1. Field of the Invention
This invention relates to a shield-plated corrugated tube, and particularly to a shield-plated corrugated tube which has a tube wall corrugated by repeating convex-concave portions in the axial direction of the tube to thereby be flexible, and which is designed to cover a bundle of electric wires or the like to perform mechanical protection and electromagnetic shielding of the bundle of electric wires or the like accommodated therein.
2. Background
Recently, controllers for cars, machine tools, office machines, etc. have been made with electronics, and countermeasure of these electronic equipments against electromagnetic-wave noise has become a serious problem. In the electronic equipments, wires connected thereto are apt to absorb electromagnetic-wave noise, so it is necessary to have a shield-countermeasure to shield internal conductors of the wires from external electromagnetic waves. Shielding the internal conductors is performed by grounding external conductors covering the internal conductors.
One of the external conductors used in this occasion is a shield-plated corrugated tube in which plating is applied on a flexible and bellows-like tube (corrugated tube) made from resin material. The shield-plated corrugated tube is obtained by plating a corrugated tube, originally used as a wiring protector, in order to make the corrugated tube also have a shielding effect. In the shield-plated corrugated tube, a slit for insertion of wires is formed in the axial direction (longitudinal direction) of the tube in the same manner as in a conventional corrugated tube.
FIG. 7 shows a structural example of such a conventional shield corrugated tube.
In this shield corrugated tube 1, an electromagnetically shielding metal layer 5 is provided on the inner surface of a corrugated tube 3 formed from insulating resin material and having flexibility.
As illustrated, the corrugated tube 3 has a structure in which a tube wall is formed into a corrugated (bellows-like) shape so as to repeat predetermined convex-concave portions in the axial direction of the tube, and which has a slit 7 parting the tube wall in the axial direction of the tube. If the slit 7 is opened and the corrugated tube 7 is made to cover a bundle of electric wires or the like, the bundle of electric wires or the like can be accommodated therein easily, and mechanical protection and electromagnetic shielding of the accommodated bundle of electric wires or the like can be attained.
Examples of the method of providing such a metal layer include a method in which a corrugated pipe formed of metal is used as the metal layer, and the surface of the metal pipe is coated with an insulating resin layer; a method in which metal such as aluminum is evaporated onto a corrugated tube which has been made from insulating resin in advance; a method in which an electrically conductive paint is applied onto on a corrugated tube which has been made from insulating resin in advance; and so on.
However, in the above mentioned method using a metal pipe, it is difficult to form a coating of an insulating resin layer, so that there arise a problem that the cost is increased, and a problem that the flexibility is poor.
In the method by using evaporation, since it is impossible to make the metal layer thick, it is difficult to make the performance of shielding high. In addition, since there occurs a difference in the thickness of the metal layer between the vertical surface and the horizontal surface in the convex-concave portions, there arises a problem with the unstable shielding performance.
In the method in which electrically conductive coating is applied in the form of paint, it is difficult to perform uniform application of paint, so that there is a problem of unstable shielding performance. Further, this method is not suitable for manufacturing a long shielded corrugated tube.
Electromagnetic shielding is attained by the electromagnetic wave reflection and absorption functions of a metal layer. However, in each of the above-mentioned methods, a thin metal layer can be formed on only either one of the inner and outer surfaces of a corrugated tube, and particularly the effect of reflection can be obtained only once by one sheet of the thin metal layer. Accordingly, there is a basic problem that it is difficult to obtain a sufficient electromagnetically shielding effect.
Under such a background, it has been therefore considered that such metal layers are formed on both the inner and outer surfaces of a corrugated tube made from insulating resin by employing the method of electroless plating. However, in the case where the electroless plating method is employed, bubbles generated during a plating process are collected in concave portions in the corrugated tube, or a plating solution remains, so that it is easy to produce a failure in plating.
Therefore, it is regarded as a problem to be solved in the future to prevent bubbles from being collected in concave portions in the corrugated tube during a plating process, to prevent a plating solution from remaining, and so on.
It is therefore an object of the present invention to solve the foregoing problems.
It is another object of the present invention to provide a shield-plated corrugated tube in which metal layers uniform in thickness can be formed on both the inner and outer surfaces of a corrugated tube by employing an electroless plating method, a superior electromagnetically shielding effect can be obtained by the metal layers formed on both the inner and outer surfaces of the corrugated tube, and the flexibility which is important in handling can be ensured satisfactorily.
In order to attain the foregoing objects, according to the present invention, provided is a shield-plated corrugated tube for covering a bundle of electric wires or the like to mechanically protect and electromagnetically shield the bundle of electric wires or the like accommodated therein, including: a flexible corrugated tube formed from insulating resin material so as to have a tube wall which is corrugated by repetition of predetermined convex-concave portions in an axial direction of the tube; and an electromagnetic shielding metal layer disposed on a surface of the corrugated tube, in which the size of the corrugated tube is set at least so that each of grooved portions for forming the convex-concave portions satisfies a condition of 0.5 less than D/W less than 1.6 where D represents a depth of each groove in the radial direction of the tube, and W represents a width of each groove in the axial direction of the tube, and wherein the metal layer is formed by electroless plating on each of inner and outer surfaces of the corrugated tube.
According to the study of the inventor of the present application, it was made clear that whether bubbles generated in a plating process are apt to be collected in concave portions in the corrugated tube or not shows a close relation with the shape of grooved portions inside the tube for forming convex-concave portions which make the tube wall of the corrugated tube wavy when the metal layers are formed on both the inner and outer surfaces of the corrugated tube by an electroless plating method.
For example, in the grooved portions inside the tube for forming the convex-concave surfaces, assume that the depth of each groove in the radial direction of the tube is D, and that the width of the groove in the axial direction of the tube is W. Then, if the value of D/W was 1.5 or less, no accumulation of bubbles was detected in all the examined samples, and when the value of D/W was 1.6, one accumulation of bubbles was detected in ⅓ of the samples. When the value of D/W was 1.7 or more, there was such a tendency that the number of generated accumulations of bubbles was increased suddenly.
In addition, the inventor of the present application inquired the correlation between the above-mentioned value of D/W and the flexibility of the corrugated tube with respect to corrugated tubes of various nominal diameters. As a result, it was found that when the value of D/W is 0.5 or less, a large bending radius R is required, and the flexibility is lowered.
Therefore, when a corrugated tube is set at least so that grooved portions inside the tube formed into a shape having an approximately rectangular section and satisfying 0.5 less than D/W less than 1.6 as in a shield-plated corrugated tube according to the present invention, no accumulation of bubbles is generated in an electroless plating process, and metal layers uniform in thickness can be formed on both the inner and outer surfaces of the corrugated tube, so that a superior electromagnetically shielding effect can be obtained by the metal layers formed on both the inner and outer surfaces of the corrugated tube. Further, it is possible to ensure satisfactory flexibility which is important in handling.